code: def _recursive_expression(x, order, ops):
      """Generate a list of operations using the
      recursive expression which defines the Trotter product.
  
      Args:
          x (complex): the evolution 'time'
          order (int): the order of the Trotter expansion
          ops (Iterable(~.Operators)): a list of terms in the Hamiltonian
  
      Returns:
          list: the approximation as product of exponentials of the Hamiltonian terms
      """
      if order == 1:
          return [qml.exp(op, x * 1j) for op in ops]
  
      if order == 2:
          return [qml.exp(op, x * 0.5j) for op in ops + ops[::-1]]
  
      scalar_1 = _scalar(order)
      scalar_2 = 1 - 4 * scalar_1
  
      ops_lst_1 = _recursive_expression(scalar_1 * x, order - 2, ops)
      ops_lst_2 = _recursive_expression(scalar_2 * x, order - 2, ops)
  
      return (2 * ops_lst_1) + ops_lst_2 + (2 * ops_lst_1)
  
file_path: pennylane/pennylane/templates/subroutines/trotter.py
imports: import pennylane
  from pennylane.operation import Operation
  from pennylane.ops import Sum
label_desc: Use image node_id 2 for calling a global function with example usage: _recursive_expression(x, order, ops) and returns: unknown, unknown, unknown